This invention generally relates to welding flange nuts to other objects and, more specifically, to a method of welding a flange nut to a workpiece by directing current flow through weld projections on the flanged portion of the nut. An improved electrode design facilitates use of the method.
Flanged nuts have integral washers that simplify handling. They are useful in assembling components using a bolt fastener and the flange can bridge an oversized hole in an attached object. Flange nuts are commonly pre-attached to an object so that the object can be bolted to something else. This method of attachment is useful in joining automotive body parts using welded flange nuts, which facilitates the assembly of the vehicle.
Projection welding is a common practice for joining a flange nut to a metal sheet or other workpiece. Integral projections on the metal sheet contacting side of the flange provide fusible metal for the weld. Two counter-acting welding electrodes with flat contact surfaces hold the flange nut and workpiece together at a joining region with the projections on the flange pressing against the surface of the workpiece. A controlled electrical power source delivers a pulse of high amperage AC (or rectified AC) current through the facing electrodes, through the interposed nut, and the workpiece. Typically a 60 Hertz welding current is applied for several cycles of electrical current application (i.e., a fraction of a second). Although the entire nut and underlying workpiece area are heated, the higher density and momentary current flow through the projections tend to selectively melt the projections. When the current flow is stopped, the melt solidifies and the projections are fused to the workpiece.
Currently, flange nut welding processes use a copper welding electrode with a flat, full faced electrode tip to engage the top of the flange nut. The flat welding electrode tip is large enough to fit flange nuts of different sizes and it is effective in applying clamping pressure on the flat surface of the top of the nut""s stem. However, the current flow through the stem may be sufficient to soften it during the brief welding period and the hot electrode can stick to the workpiece. The counteracting electrode, which engages the opposite side of the workpiece, usually experiences no such sticking.
Thus, it is an object of the present invention to provide an improved method for welding a flange nut to a workpiece, such as a sheet metal part. The purpose of the method is to avoid sticking of nut metal to the electrode and thus to increase the reliability of the process in production and to increase the useful life of the electrode tip. It is a further object of the present invention to provide an improved electrode tip design for contacting the flange nut during welding and to facilitate the practice of the process.
The present invention provides a method of welding a flange nut to a surface of a metal sheet, or other workpiece, using an electrode design for the nut that engages only the flange portion of the nut.
A flange nut, comprising a stem, a flange at one end of the stem, and a hole extending through the stem, is welded to the workpiece at a predetermined attachment location. Typically, the workpiece will have a bolt hole, or the like, at the welding location so that after attachment of the flange nut the workpiece can be bolted to another object.
The flange portion of the nut, having pre-formed underlying projections for welding, is placed against the surface of the workpiece so that the projections lie against it and separate the nut from the workpiece by the height of the projections. Opposing electrodes engage the nut and the opposite side of the workpiece, pressing them together for effective welding force and current flow. A suitable welding current is then directed to the electrodes and through the flange portion of the nut, through the weld material projections from the flange, and through the abutting workpiece. The duration of current flow is brief, typically less than a second. The current density in the relatively small projections is sufficient to fuse them without fusing the larger flange material that carried them. The molten metal from the melted projections flows between the flange and workpiece surface or is expelled under the pressure of the electrodes. When current flow is stopped the molten metal is cooled and solidified by heat flow to the cooler surrounding metal of the nut and workpiece.
The weld material projections are integral with the flange and formed of the same material. They provide sufficient metal for the weld. Often the flange is round and formed by upsetting one end of the stem of the nut. The projections are formed at the same time and may, for example, be shaped like truncated cones spaced in a circle around the bottom surface of the flange. However, in a preferred embodiment of the invention, the weld projections are tapered and spaced circularly on the bottom of the flange.
The method of the present invention preferably employs a welding electrode having an improved design for better weld quality. The electrode for engaging the flange nut comprises a recess formed in the face of the electrode tip that enables the rim of the electrode tip to fit over the stem portion of the nut without touching it. As a result, the hollow electrode tip establishes clamping pressure and electrical contact with only the flange portion of the nut. The current then passes mainly through the flange and its underlying projections to the workpiece metal. The contact of the annular electrode tip with the flange of the nut does not result in flange metal adhering (sticking) to the electrode during the projecting welding operation.
Industrial processes rely on the durability of their welding electrodes to produce hundreds of individual welds having superior weld quality without constant replacement of the electrodes. Electrode sticking usually degrades an electrode tip and impedes current flow. The method and electrode design of the present invention reduces electrode failure and welding inefficiencies by redirecting the electrical current away from the stem portion of the nut. The result is improved weld quality and improved life of the electrode tip.
These and other objects and advantages of this invention will become apparent from a detailed description of the preferred embodiment that follows.